I’m not 100% certain what the Street Triple bars have but here are picks of the ends of Tiger and Street Scrambler bars. The inserts are held in place by a pin that can be pressed or punched out. I’d check before you go “wrenching”.
The bar end mirrors I put on my Scrambler came with a spacer to go around the outside of the handle bar, leaving the insert in place to mount an end ”cap” ie disc.

 

Well here's what I found in the service manual. And so far I have taken off the bolt and cap.

 

Slip the throttle tube off and have a look at what’s there. I see the diagram is for a model with throttle cables, rather than the newer ride by wire. Is that what you have? Knowing which bike you have would make it easier to answer your question.

 

Check out this post. She deals with the exact same thing on our specific bike.

Pics included.

 

Well I took some time to look a little closer at the problem and found that I could slide the insert off the end cap and the bolt fit the threaded end of the bar and just bolted them right up and fit the mirrors on.

 

i picked up some OEM knockoff for $19 on amazon. i was able to use the existing bolt with the new bar end mirrors. worked like a charm

Amazon.com: Frenshion Compatible for 7/8" 22mm Motorcycle Handlebar End Mirrors Motorbike Aluminum Rearview Mirrors Side Mirrors Black: Automotive

Amazon.com: Frenshion Compatible for 7/8" 22mm Motorcycle Handlebar End Mirrors Motorbike Aluminum Rearview Mirrors Side Mirrors Black: Automotive

www.amazon.com